1. Field of the Invention
The present invention relates to lithography. More specifically, the present invention relates to a magnetically levitated and driven reticle-masking blade stage mechanism.
2. Related Art
The size of circuit elements that can be used in integrated circuits is a direct function of the wavelength of light used in the projection photolithography fabrication process. Thus, realization of integrated circuits with increasingly smaller features depends on the development of photolithography tools that operate with progressively smaller wavelengths.
State-of-the-art projection photolithography tools now utilize deep ultraviolet (DUV) light with wavelengths of 248 nm, 193 nm, or 157 nm. At these wavelengths available optical materials, including silicon oxide and calcium fluoride, allow the use of both transmission reticles, and refractive optical elements. Since the shorter wavelengths are absorbed by oxygen, nitrogen or helium must be used in the optical path.
Future photolithography tools may utilize extreme ultraviolet (EUV) radiation with wavelengths of 10 nm to 15 nm. At these wavelengths the reticle and all other optical elements must be reflective, and the entire optical path must be evacuated.
In a state-of-the-art projection lithography tool, the exposing light from the source optics is transmitted through, or reflected from, the patterned reticle and then imaged by projection optics onto the surface of a resist coated wafer. High-resolution projection optics have relatively small field sizes and thus require that the wafer and the reticle be scanned synchronously to exposure a full rectangular field. The reticle has an absorbing layer, usually chrome, in the border area that surrounds the circuit pattern to prevent unwanted exposure of the wafer. Additionally, a state-of-the-art projection lithography tool usually has movable reticle-masking (REMA) blades that: (1) remove the need for having a wide (i.e., expensive) border area, (2) block light that might otherwise leak through pin holes in the border area, (3) allow a selected portion of the full patterned area to be exposed, and (4) selectively block reticle alignment targets so that they are not printed on the wafer.
Generally, lithography tools use four independently movable REMA blades configured as two pairs. One pair of REMA blades has edges that are aligned parallel to the exposure scan axis. This first pair remains stationary during the exposure scan and delimits the width of the exposed field. A second pair of REMA blades has edges that are aligned orthogonal to the scan axis. This second pair moves synchronously with the reticle and delimits the length of the exposed field. Synchronous scanning can be achieved by attaching the second pair of REMA blades to the reticle stage and prepositioning them prior to the exposure scan. Unfortunately, because the second pair of REMA blades requires frequent adjustments, this approach can limit the overall throughput of the lithography tool.
For state-of-the-art DUV lithography tools, a preferred design approach is to locate the REMA blades at an image plane conjugate to the reticle rather than near the reticle itself Locating the REMA blades near the reticle results in significant blurring of the delimiting edges imaged on the wafer. By locating the REMA blades at an image plane conjugate, the edges of the REMA blades can be sharply imaged on the wafer. Because there is usually no suitable image plane conjugate near the projection optics, the image plane conjugate typically is produced by optics in the illuminator.
REMA blades typically are moved by linear stage mechanisms that are driven by linear motors and that are guided by ball bearings, roller bearings, or gas bearings. The stages that control the motion of the scanning REMA blades must have capabilities that match those of the reticle stage. The stages that control the motion of the scanning REMA blades must be characterized by a high rate of acceleration and a long life. A preferred REMA blade motion control mechanism for use in DUV tools is described in U.S. Pat. No. 6,307,619 to Galburt et al., entitled “Scanning Framing Blade Apparatus”, issued Oct. 23, 2001, which is incorporated herein by reference.
Experimental work with EUV lithography tools suggests that a preferred location for the REMA blades be in front of the reflective reticle rather than at an image plane conjugate. In contrast with DUV systems, in EUV systems the additional optics necessary to produce an image plane conjugate results in excessive attenuation of the exposing beam. Fortunately, EUV systems are also characterized by relatively low numerical aperture projection optics and the absence of a protective pellicle. These features minimize the blurring of the delimiting edges imaged on the wafer that can occur when the REMA blades are located near the reticle.
However, in EUV systems the REMA blades must operate in a high vacuum environment. This gives rise to problems in using gas bearings or conventional anti-friction bearings in the stage mechanisms. Specifically, gas bearings would have to be isolated from the high vacuum environment. Likewise, ball or roller anti-friction bearings would have to be operated without lubrication, which would reduce the life of these bearings and would allow particles to be generated that could contaminate the surface of the unprotected reticle or other optical devices. Therefore, what is needed is a REMA blade stage mechanism that can support guided motion of REMA blades in a high vacuum environment. Preferably, such a REMA blade stage mechanism would also be able to support more than one degree of freedom of motion.